1. Field of the Invention
This invention relates to electronically scanned radio frequency (RF) antennas, specifically to such antennas used in fixed and mobile subscriber terminals of wireless radio frequency communication systems.
2. Description of the Related Art
The explosive growth in demand for wireless radio frequency communications necessitates increased efficiency in use of the radio frequency spectrum. In response to the problem extensive efforts have been applied to the development of antenna systems that use some form of scanning technique to improve network performance. Multiple techniques have been demonstrated such as space-diversity combining switched/multiple-beam arrays, RF scanning arrays, and digital beam forming. U.S. Pat. No. 5,903,826 to Nowak, for example, describes a wireless communication system which uses adaptive narrow beam antennas at the subscriber end of the communication link. The technique described in Nowak however is relatively complex and expensive to produce because it requires antennas having multiple polarizations. Further, the technique described in Nowak is geared to fixed access systems, and no claims are made relative to mobile subscriber units. U.S. Pat. No. 5,303,240 to Borras et al describes a similar system but it is limited to Time Domain Multiple Access (TDMA) protocols. The system described in U.S. Pat. No. 5,430,769 to Pasiokas, et al is also similar but limited to transmission and reception of digital data because it depends on the measurement of bit transition times. Each of the described techniques is based on the premise that a more directive beam scanned over a wide angle will result in reduced mutual interference thereby improving system performance for both coverage and capacity. These systems are generally referred to as smart or adaptive antennas that change radiation pattern in response to a changing signal environment.
Implementation of smart antennas at the base station of wireless systems provides narrow beams to be generated for each subscriber or group of subscribers. Consequently, the smart antenna reduces interference by forming nulls in the direction of other sources, thereby improving system capacity and coverage. See, for example, U.S. Pat. No. 5,907,816 to Edward M. Newman et al. The techniques described in Newman's patent also involve forming several narrow antenna beams to improve coverage of the base station. However, the techniques described are not applied at subscriber units. Despite all efforts to date, no subscriber based smart antenna system has been widely accepted primarily because of a failure to produce a cost effective device capable of supporting the large number of fixed and mobile subscribers found within a typical cellsite coverage area. While smart antennas have been applied at base stations, their use is limited due to high cost.
One alternative solution to improve system performance by reducing interference is to provide a stationary highly directive antenna with each subscriber unit. Such a solution has its obvious limitations for mobile subscriber applications stemming from the fact that mobility of the subscriber unit would frequently result in the antenna beam being directed away from the base station transmitting the optimal signal. However, this technique has been implemented in fixed wireless applications in which the subscriber unit is stationary. The solution utilizes a highly directive antenna such as a Yagi-Uda mounted on a roof top for each subscriber unit. The antenna is mounted with the main beam directed at the base station with the strongest signal. Mounting of the antenna requires specialized labor making this a costly solution. Furthermore, this solution is not adaptive to a growing wireless network where increased capacity requires addition of cellsites resulting in fixed subscriber antennas that are no longer directed toward the optimal base station.